In many situations, such as military or security applications, it is desirable to have communication systems with modes supporting low probability of detection (LPD). LPD communication systems may provide users with a tactical advantage or enhanced safety relative to other communication systems, such as through increasing difficulty of a non-member of the system to acquire and recognize signal transmissions. A significant challenge for LPD-supporting communication systems is to operate at extremely low signal-to-noise ratios (SNR), including SNR of well below negative 20 decibels (dB). For instance, LPD communication systems must be capable of distinguishing low power signal transmissions from background “noise” in the system.
There are typically two fundamental parts of receiving a signal transmission: (1) detecting the signal transmission presence and (2) receiving the data portion of the signal transmission. In general, detecting the signal transmission presence is accomplished by including some known sequence of bits at a known position of a transmission (for example, typically in the first few bits). These known sequences of bits sometimes may be referred to as preambles, since they generally precede a data or message portion of a signal transmission. A communication system receiver correlates received signal information against the known sequence and uses information gathered to establish timing and Doppler estimates for the signal message. The estimated timing information is then used to demodulate the data portion of the message.
The two fundamental parts of receiving a signal transmission may differ in that signal transmission presence detection occurs via acquisition hardware of a communication system. The acquisition hardware searches for a message with any Doppler/timing offset. Whereas receiving the data portion involves data demodulation that demodulates the signal at a given time (typically after the signal transmission presence detection is completed).
In most communication systems, detecting signal transmission presence is easily accomplished. For example, a power measurement on the communication channel may be sufficient to detect a signal transmission. Communication systems that handle slightly lower SNRs are able to run acquisition correlators that continuously search over all time offset/Doppler combinations to locate acquisition preambles. However, the hardware required for this type of acquisition grows roughly linearly with the length of the acquisition preamble, making it infeasible for the very long acquisition preambles needed at very low SNRs. Thus, it is desirable for a communication system to operate at very low SNRs while reducing the hardware required for successful operation.